Conventionally, with optical modulators that modulate light generated by a light source, a Mach-Zehnder interferometer may be used. In recent years, because optical modulation methods are diversified, each optical modulator is often provided with two or more Mach-Zehnder interferometers. In those situations, by integrating the two or more Mach-Zehnder interferometers on one chip, it is possible to keep the size of the optical modulator small.
An optical modulator provided with one or more Mach-Zehnder interferometers includes a signal electrode and a ground electrode provided along parallel optical waveguides. More specifically, an optical modulator may be provided with one or more Radio Frequency (RF) electrodes for high frequency signals used for multiplexing data onto light and one or more Direct Current (DC) electrodes for direct-current signals used for controlling the phase of the light and monitoring temperatures. These electrodes are each connected to a lead pin, so that electrical signals are input thereto from the lead pins. When an optical modulator is provided with two or more Mach-Zehnder interferometers, a larger number of electrodes are used, which also increases the number of lead pins being used.
The lead pins of an optical modulator may be connected to other component parts (e.g., a Large Scale Integration (LSI) circuit), by using a Flexible Printed Circuits (FPC) unit that has flexibility. In other words, for example, the lead pins of the optical modulator may be soldered onto one end of the FPC unit, while the other end of the FPC unit is inserted into a connector connected to the LSI circuit or the like. In that situation, the one end of the FPC unit onto which the lead pins are soldered has conductor pattern lands formed thereon in correspondence with the lead pins. In contrast, the other end of the FPC unit inserted into the connector has conductor pads formed thereon in correspondence with the lead pins, so that the pads are in contact with terminals provided inside the connector. Further, mutually-corresponding lands and pads are connected to one another by wiring patterns formed on the FPC unit.
Patent Document 1: Japanese Laid-open Patent Publication No. 2007-234500
Patent Document 2: Japanese Laid-open Patent Publication No. 2010-278132
Incidentally, as for positional arrangements, the terminals provided inside the connector and the lead pins of the optical modulator are not necessarily always arranged in a mutually-corresponding order. Consequently, on the FPC unit, the sets of lands and pads to be connected together are not arranged in the order of connection. Thus, the wiring patterns used for connecting the lands and the pads to one another may intersect one another. In that situation, because it is difficult to cause the wiring patterns to intersect one another on a single surface of the FPC unit, one of two intersecting wiring patterns may be formed on the front surface of the FPC unit, while the other of the two wiring patterns may be formed on the rear surface of the FPC unit. With this arrangement, it is possible to arrange the mutually-corresponding lands and pads to be connected to one another on the FPC unit. As a result, it is possible to connect the mutually-corresponding sets together that are made up of the terminals provided inside the connector and the lead pins of the optical modulator.
However, because all the terminals provided inside the connector are arranged on a single plane, in order to bring the pads formed on the FPC unit into contact with the terminals provided inside the connector, all the pads need to be formed on a single surface of the FPC unit. In other words, for example, when the front surface of the FPC unit is brought into contact with the terminals provided inside the connector, the pads that are connected to the wiring patterns formed on the rear surface of the FPC unit also need to be provided on the front surface of the FPC unit. For this reason, as for the wiring patterns formed on the rear surface of the FPC unit, for example, pads may be provided in such a manner that conductor patterns formed on the two surfaces of the FPC unit are connected to each other via a through hole. In contrast, as for the wiring patterns formed on the front surface of the FPC unit, for example, it is possible to use the conductor patterns themselves formed on the front surface of the FPC unit as pads, without any extra arrangement.
However, if the structures of the pads vary depending on whether the corresponding wiring pattern is formed on the front surface or the rear surface of the FPC unit, the thickness of the FPC unit varies among different parts corresponding to the different pads. Thus, a problem arises where the electrical conduction between the FPC unit and the connector becomes unstable. In other words, in the example described above, the pads for the wiring patterns formed on the rear surface of the FPC unit are provided in such a manner that the conductor patterns are formed on the two surfaces of the FPC unit and that these conductor patterns are connected to each other via the through hole. For this reason, those pads are thicker than the pads for the wiring patterns formed on the front surface of the FPC unit. Thus, there is a possibility that a contact failure may occur between the pads for the wiring patterns formed on the front surface of the FPC unit and the terminals provided inside the connector. As a result, the electrical conduction between the FPC unit and the connector becomes unstable.